Natural mineral formations that contain tungsten in various compounds and industrial concentrations, when extraction is technically possible and economically feasible - tungsten, molybdenum in ores, as well as beryllium, tin, copper, bismuth, occasionally mercury, antimony, silver, gold, arsenic, tantalum, sulfur, scandium, niobium - the planet, judging by the name of their group, is not rich in such rare-earth metals. An associated component of tungsten ore, molybdenum, like most others, is extracted during enrichment and converted into selective or collective concentrates.
How did tungsten come about?
Swedish chemist Karl Scheele, a pharmacist by training, conducted experiments in his own laboratory. There he discovered manganese, barium, chlorine, and even oxygen for humanity. All his life he did nothing but make discoveries, for which he was accepted into the Stockholm Academy of Sciences. And even shortly before his death in 1781, he did not stop doing what he loved, thus giving us another wonderful gift.
Carrying out an experiment, Karl Scheele discovered that tungsten (a mineral later named scheelite in his honor) was a salt of some still unknown acid. This was a huge discovery, but only two years later, chemists from Spain and his students isolated a completely new element from this mineral, which turned all the postulates in the industry upside down. However, this revolution did not happen immediately; a century passed before it became clear what exceptional properties tungsten has.
Metals of group VI of the secondary subgroup (Cr, Mo, W)
All three metals of the secondary subgroup of group VI - chromium, molybdenum and tungsten - were discovered one after another at the end of the 18th century.
Chrome (Chromium)
It was discovered in 1766 in the mineral crocoite PbCrO4 by the Russian mineralogist I. G. Leman. The metal was first isolated in its free form in 1797 by the French chemist Louis Nicolas Vauquelin.
Chromium
Chrome is a silver-white metal, hard and refractory (tmelt = 1890°C). At room temperature it does not oxidize in air. The metal ingot is resistant to oxidation even at elevated temperatures, while chromium powder burns in air already at 300 °C, forming green chromium (III) oxide Cr2O3 . Dilute sulfuric and hydrochloric acids dissolve chromium, releasing hydrogen. Chromium is insoluble in cold concentrated nitric acid and is passivated after treatment with it.
Chromium is a fairly common element. The earth's crust contains approximately the same amount as chlorine and vanadium - 0.02%. chromium iron ore predominates . When it is reduced with coal, ferrochrome is formed - an alloy of chromium with iron: FeCr2O4 + 4C = Fe + 2Cr + 4CO. To obtain pure metal, chromium iron ore is purified from iron, transferring chromium to the highest oxidation state (+6), and only then it is reduced with aluminum.
Ferrochromium (containing about 60% chromium) and pure chromium are used as additives to steels: the resulting alloy - chromium steel - is highly hard and resistant to oxidation. The most popular grade of stainless steel contains 18% chromium and 6% nickel. This steel is used in the chemical and petrochemical industries, as well as for the manufacture of household appliances. Alloys of chromium and nickel - nichrome - are not only resistant to corrosion, but also have high electrical resistance. They are used to make incandescent coils for electric heating devices. Many products are coated with a thin layer of chrome - chrome plated. The chrome coating gives the product a beautiful appearance and protects it from corrosion.
Chromium(III) chloride
The name “chrome” (from the Greek “chrome” - “color”, “paint”), proposed by Vauquelin, emphasizes the variety of colors of the salts of this metal. Thus, in aqueous solutions, chromium (II) compounds have a sky blue color, chromium (III) - violet or green, chromium (VI) - orange or yellow.
When oxidized in non-oxidizing acids, such as hydrochloric acid, chromium goes into the +2 oxidation state:
Cr + 2HCl = CrCl2 + H2↑
But the resulting light blue solution is stable only in the absence of oxygen. In air it instantly turns green as chromium oxidizes:
4CrCl2 + 4HCl + O2 = 4CrCl3 + 2H2O
If II ) compounds exhibit basic properties, then trivalent chromium compounds are amphoteric. III ) hydroxide Cr ( OH )3 , precipitated from a solution of chromium salt under the action of ammonia:
CrCl3 + 3NH3∙H2O = Cr(OH)3↓ + 3NH4Cl,
is an amphoteric base. When it is dissolved in acids, chromium (III) salts are formed:
2Cr(OH)3 + 3H2SO4 = Cr2(SO4)3 + 6H2O,
and in caustic alkalis - hydroxochromates (III) of alkali metals (they are also called chromites):
Cr(OH)3 + 3NaOH = Na3[Cr(OH)6].
Potassium chromate
In an acidic environment, chromium compounds ( III ) are quite resistant to oxidation, but in the presence of alkali they are easily oxidized to chromates:
2Na3[Cr(OH)6] + 4NaOH + 3Br2 = 2Na2CrO4 + 6NaBr + 8H2O.
Chromates are yellow salts, derivatives of chromic acid H2CrO4, which is stable only in very dilute solutions. Alkali metal chromates are highly soluble in water. In an acidic environment they turn into orange dichromates - salts of dichromic acid H2Cr2O7:
2Na2CrO4 + H2SO4 = Na2SO4 + Na2Cr2O7 + H2O
When alkalized, the dichromate turns back into chromate:
Na2Cr2O7 + 2NaOH = 2Na2CrO4 + H2O, and the solution turns yellow again.
In an acidic environment, dichromates are strong oxidizing agents. The products of their reduction are Cr3+ ions:
K2Cr2O7 + 4H2SO4 + 3K2SO3 = Cr2(SO4)3 + 4K2SO4 + 4H2O
At a low temperature, it is possible to isolate violet crystals of potassium chromium alum KCr(SO4)2∙12H2O .
The dark red solution obtained by adding concentrated sulfuric acid to a saturated aqueous solution of potassium dichromate is called " chrompic ". In laboratories it is used for washing and degreasing chemical glassware. The dishes are carefully rinsed with chromium, which is not poured into the sink, but is used repeatedly until the mixture turns green - all the chromium in such a solution has already converted into the Cr3+ form.
Potassium dichromate
A particularly strong oxidizing agent is chromium ( VI ) CrO 3 . With its help, you can light an alcohol lamp without matches: just touch the wick moistened with alcohol with a stick containing several crystals of this substance.
By decomposing CrO3, dark brown chromium(IV) oxide powder CrO2 can be obtained. It has ferromagnetic properties and was formerly used in the magnetic tapes of some audio cassettes.
The adult human body contains only about 6 mg of chromium. Many compounds of this element (especially chromates and dichromates) are toxic, and some of them are carcinogenic.
Molybdenum (Molybdaenum) and tungsten (Wolframium)
Black crystals of molybdenum disulfide MoS2 with a metallic sheen were known to people in ancient times. This substance was often confused with graphite or PbS galena. The name of the mineral “molybdenite” emphasized the similarity of its mechanical properties with the properties of lead (translated from Greek “molybdos” means “lead”): it could be used for writing on parchment. In 1778 K.V. Scheele proved that molybdenite does not contain lead and isolated the oxide of a new element from it. A few years later, the Swedish chemist P. Hjelm, calcining this oxide with coal, obtained the metal. Based on the mineral it contains, the new element began to be called molybdenum.
Molybdenum coin
Around the same time, in 1781, Scheele isolated another metal oxide from the mineral tungsten (its current name is scheelite CaWO4). Two years later, Spanish chemists brothers Fausto and Julio de Elujar obtained it in the form of a simple substance by calcining another mineral with coal - wolframite (Fe, Mn)WO4. It is interesting that in some languages, for example in English, the name tungsten is assigned to the element. And the word “tungsten” is derived from the German words “Wolf” and “Rahm” and literally translates as “wolf foam”. This is how some tungsten minerals were called in the Middle Ages, which interfered with the smelting of tin. They converted the metal into slag in the form of tin tungstates - “they devoured the tin like a wolf devouring a sheep,” as stated in one of the medieval treatises on metallurgy.
Molybdenum (tmelt = 2615оС) and tungsten (tmelt = 3410оС) are among the most refractory substances. The high hardness inherent in these metals, combined with heat resistance, makes them indispensable in the production of high-temperature structural materials. It is not without reason that tungsten is used to make the filaments of electric lamps (the filament heats up above 2500°C) and the cathodes of X-ray tubes, and the alloy of molybdenum and titanium can be used at temperatures up to 1500°C. Alloys based on tungsten carbide WC are particularly hard. They are used to make cutting parts of tools and drills.
Molybdenum and tungsten are much less chemically active than chromium. Acids and alkalis have practically no effect on them. The exception is hot fuming nitric acid, which slowly oxidizes metals to the highest oxidation state (+6):
W + 6HNO3 = H2WO4↓ + 6NO2 + 2H2O
The resulting precipitates of H2MoO4 and H2WO4 are called molybdic and tungstic acids, respectively, since under the action of alkalis they give salts - molybdates (for example, Na2MoO4) and tungstates (Na2WO4).
Tungsten
When molybdic and tungstic acids are heated, the corresponding higher oxides are formed - MoO3 and WO3. By reducing them with hydrogen at 1100°C, metal powders are obtained: WO3 + 3H2 = W + 3H2O.
Converting such a powder into an ingot is not an easy task, because both metals are extremely refractory. To do this, the metal powder is mixed with a solution of glycerin in alcohol and small pieces are pressed from the resulting mass. Then they are heated to a high temperature (the glycerin burns out) and an electric current is passed through them. Under the influence of the released heat, individual microcrystals are fused with each other, forming a compact ingot.
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*in the image of the record the mineral is crocoite
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Separation
Depending on the deposit, all tungsten ores are divided into two types: exogenous and endogenous. Among the latter are skarn, pegmatite, vein-vein (hydrothermal), grazer types of genetic ores, which are combined into three main ore formations. These are tungsten - tin, tungsten - molybdenum, tungsten - polymetals.
Sometimes tungsten is found in pegmatites, from where both it and scheelite are extracted along the way in the mining of beryl, cassiterite, tantalum, niobates or spodumene. Pegmatite deposits—the sources of alluvial placers—are most developed in Southeast Asia and Africa.
Reserves
Tungsten and molybdenum in ores are closely associated with granite intrusions, their apical parts, where top deposits are observed, quite often accompanied by ore stockworks, both intra- and supra-intrusive.
They are cloak-shaped deposits in shape, isometric and oval with mostly gentle bedding. Columnar ore bodies and irregularly shaped stockworks were also noted. Deposits containing molybdenum, tungsten and other rare earth minerals almost never have large reserves. The ore is estimated at only tens, very rarely hundreds of thousands of tons.
Molybdenum production
Molybdenum is produced by oxidative roasting of standard molybdenite concentrates, in which the content of pure substance, excluding the impurities present, reaches 47-50%. During this process, the temperature is maintained in the region of 570-6000C. For this purpose, multiple hearth furnaces or fluidized bed furnaces are used.
However, this can be said to be the finish line on the path to obtaining the substance. The preceding stage is ore enrichment using the flotation method. Before flotation, the ore is crushed. This process allows the output to be a concentrate containing about 10% of the substance. After this, the next stage of flotation of the already obtained concentrate is carried out - selective, in which special reagents are used, as a result of which, through selective separation from other sulfides, it is possible to obtain MoS2. To obtain high-quality molybdenum concentrate, the molybdenum content of which reaches 48-58%, the entire production cycle is repeated 5-6 times, using intermediate grinding.
The main place among all stages of chemical processing of concentrate is given to roasting, which makes it possible to remove unwanted impurities in the form of sulfur, water, as well as residual particles of flotation reagents. If rhenium is present in the concentrate, during firing, volatile oxide Re2O7 is obtained, which is removed along with the furnace gases.
Thus, during firing, molybdenum disulfide is oxidized to trioxide 2MoS2 + 7O2 = 2MoO3 + 4SO2. In addition, many side reactions also occur during the process, which have a significant impact on the further production of molybdenum:
- 6CuFeS2 + 19O2 = 2Fe3O4 + 6CuO + 12SO2
- MoO3 + CaCO3 = CaMoO4 + CO2
- MoO3 + CuO = CuMoO4
- MoO3 + PbO = PbMoO4.
How effective roasting will be is influenced by a number of factors, the main of which is the degree of grinding of the concentrate.
The cinder obtained containing molybdenum anhydride is converted into ammonium paramolybdate or pure MoO3, which subsequently makes it possible to obtain any molybdenum compounds, even with maximum purification.
When producing metal, the ammonia method is also popular, the essence of which is to dissolve molybdenum anhydride in 8-10% aqueous ammonia. At the same time, most of the impurities contained in the cinder remain undissolved. This method, under certain conditions, allows you to obtain about 80-95% molybdenum. The part of MoO3 that remains in the cinder is further processed.
Another way to obtain metal is sublimation from cinders at temperatures of 900-10000C. Through this method it is possible to obtain 99.95% pure MoO3.
To obtain metal molybdenum, a reduction method is used, when the reduction of MoO3 occurs in a flow of dry hydrogen. For this process, special tubular furnaces are used, the temperature in which reaches 7000C at the first stage of reduction, and 10000C at the second.
Production
Molybdenum, tungsten and other hydrothermal ores are located in the zones of exo- and endocontact of granite massifs, which form quite extensive series of steeply dipping veins, up to a kilometer in depth; average dip veins are much less common. Stockworks are also found. Ore bodies consist of quartz-wolframite-cassiterite, quartz-wolframite inclusions, often with molybdenum, beryl and bismuthine, interspersed with quartz-molybdenite-scheelite or quartz-scheelite ores.
Typically, such ores contain tungsten, molybdenum, and other rare earth metals in small quantities: tungsten from half a percent to one and a half percent, more often less. And this is with ore reserves of several thousand or several tens of thousands of tons, which is also very, very small. Mining is usually done underground or open pit.
How it works
The principle of operation of molybdenum additives can be briefly described as follows.
- During a scheduled oil change, the required amount of additive must be poured into the crankcase compartment. Each motor and fluid modification will require its own proportions. The dosage should be found in the manufacturer's instructions.
- While the engine is running, a chemical reaction occurs and the additive components are mixed with the oil. Next, a connection is formed that protects the surfaces of the parts from direct contact. This increases their service life.
An important parameter is that additives only work if they are used continuously. Therefore, every time you change the oil, you need to fill in a new bottle of fluid. Otherwise, all the positive effect will come to naught. The driver will also be required to strictly adhere to the rules for using additives. If the technology is broken, there may not be a positive effect.
Market specialists and experienced drivers advise paying attention to the manufacturer and customer reviews when selecting additives. You can often find counterfeits or low-quality products on store shelves.
Extraction methods
Tungsten deposits involve mining methods either by caving in layers or by horizontally layering the ore in mined blocks. The method of filling mined-out space is also used, which is good when developing veins, skarn or greisen deposits.
The open method involves the presence of curtainwalls, skarn or greisen deposits or placers. In quarries where tungsten and molybdenum ore are mined, there is usually a transport system and external dumping. In these cases, production is almost completely mechanized—ninety-five percent. But the work doesn't end here. Ores require beneficiation, since they contain only a maximum of one and a half percent of rare earth metals - tungsten, molybdenum.
Place of Birth
In the territory of the former USSR, the most significant deposits of tungsten ore have been explored in Kazakhstan, Eastern Siberia and the Far East, the Caucasus and Central Asia. Not all of them are being developed. Abroad, processing of tungsten and molybdenum is carried out especially in South Korea and China. The most significant deposits in the world are located there. In addition, tungsten is mined in Portugal, Australia, Canada, Bolivia, the USA, France, Austria and Turkey.
It must be said here that Southeast Asia and its Pacific ore belt have more than sixty percent of all tungsten reserves on earth. In total, in the explored deposits of the planet, the total reserves of tungsten are much less than one and a half million tons. For example, about 4,278,200 tons of gold are mined annually (not in reserves, but rather put into use).
Properties
Being one of the most refractory metals, tungsten becomes literally indispensable in all areas that involve high temperatures. As a chemical element, Wolframium (W) is in the fourth group of the periodic table. Its atomic mass is 183.85, and its number is 74. It received its name due to its light gray color - from German Wolf and Rahm are translated as “wolf” and “cream”, if literally “wolf foam”. Despite its refractoriness, it is stable at ordinary temperatures. The minerals that supply tungsten are scheelite and wolframite.
Tungsten is one of the most important components of super-hard heat-resistant steels - high-speed and tool steels, as well as alloys with the same properties - stellite, victorious, and so on. But we see pure tungsten every day, since it is widely used in electrical engineering. For example, in incandescent lamps, tungsten filaments. It is also indispensable in radio electronics. Electronic devices have cathodes and anodes made of this metal.
Chemical properties of Molybdenum.
Molybdenum is stable in air at ordinary temperatures.
The onset of oxidation (tarnish color) is observed at 400 °C. Starting at 600 °C, the metal quickly oxidizes to form MoO3. Water vapor at temperatures above 700 °C intensively oxidizes Molybdenum to MoO2. Molybdenum does not react chemically with hydrogen until it melts. Fluorine acts on Molybdenum at ordinary temperatures, chlorine at 250 ° C, forming MoF6 and MoCl6. When exposed to sulfur and hydrogen sulfide vapors above 440 and 800 °C, respectively, disulfide MoS2 is formed. With nitrogen, Molybdenum forms a nitride (probably Mo2N) above 1500 °C. Solid carbon and hydrocarbons, as well as carbon monoxide (II) at 1100-1200 °C interact with the metal to form Mo2C carbide (melts with decomposition at 2400 °C). Above 1200 °C, molybdenum reacts with silicon, forming silicide MoSi2, which is highly stable in air up to 1500-1600 °C (its microhardness is 14,100 Mn/m2). Molybdenum is somewhat soluble in hydrochloric and sulfuric acids only at 80-100 °C. Nitric acid, aqua regia and hydrogen peroxide slowly dissolve the metal in the cold, quickly when heated. A good solvent for Molybdenum is a mixture of nitric and sulfuric acids. Tungsten does not dissolve in a mixture of these acids. Molybdenum is stable in cold alkali solutions, but somewhat corrodes when heated. The configuration of the outer electrons of the Mo atom is 4d55s1, the most characteristic valence is 6. Compounds of 5-, 4-, 3- and 2-valent Molybdenum are also known. Molybdenum forms two stable oxides - MoO3 (white crystals with a greenish tint, melting point 795 °C, boiling point 1155 °C) and MoO2 (dark brown). In addition, intermediate oxides are known that correspond in composition to the homologous series MonO3n-1 (Mo9O26, Mo8O23, Mo4O11); all of them are thermally unstable and decompose above 700 °C to form MoO3 and MoO2. MoO3 oxide forms simple (or normal) Molybdenum acids - monohydrate H2MoO4, dihydrate H2MoO4·H2O and isopolyacids - H6Mo7O24, HMo6O24, H4Mo8O26 and others. Salts of normal acids are called normal molybdates, and salts of polyacids are called polymolybdates. In addition to those mentioned above, several molybdenum peracids are known - H2MoOX (x - from 5 to and complex heteropolycompounds with phosphoric, arsenic and boric acids. One of the common salts of heteropolyacids is ammonium phosphoromolybdate (NH4)3[P(Mo3O10)4] 6H2O. From Molybdenum halides and oxyhalides are most important fluoride MoF6 (melt 17.5 °C, boiling point 35 °C) and chloride MoCl5 (melt 194 °C, boiling point 268 °C). They can be easily purified by distillation and are used to obtain high-purity Molybdenum The existence of three Molybdenum sulfides has been reliably established - MoS3, MoS2 and Mo2S3. The first two are of practical importance. MoS2 disulfide occurs in nature in the form of the mineral molybdenite; it can be obtained by the action of sulfur on Molybdenum or by fusing MoO3 with soda and sulfur. The disulfide is practically insoluble in water, HCl, diluted H2SO4. Decomposes above 1200 ° C to form Mo2S3. When hydrogen sulfide is passed into heated acidified solutions of molybdates, MoS3 is deposited.
Alloy grades
Recycling tungsten and molybdenum is complex but extremely rewarding. The industry knows several brands, some of which are more common and some less common. Tungsten can be pure, with additives, and in alloys with other metals. Thus, there are different brands of VR - an alloy of tungsten and rhenium; VL - with lanthanum oxide as an additive; VI - with yttrium oxide; VT - thorium oxide as an additive; VM - with silicon-alkaline and thorium additives; VA - with silicon-alkaline and aluminum additives; HF - pure tungsten.
Tungsten serves as the basis for hard alloys, and the alloy of tungsten and molybdenum is heat-resistant, like some others. Also, with his participation, wear-resistant tool steel is prepared. Many engine parts - aviation and space - are made from such alloys; in electric vacuum devices - various parts and filaments. Since the density of this metal is very high, it is used for counterweights, for bullets and artillery shells, for ballistic missiles (flight stabilization, tungsten can withstand all one hundred and eighty thousand revolutions per minute); for ultra-high-speed rotors, metals such as tungsten and molybdenum are also used. Their application, as we see, is very wide and even, one might say, sophisticated.
Areas of application
Today neither medicine nor nuclear physics can do without these rare earth metals, such as chromium, molybdenum, and tungsten. Single crystals of all tungstates serve as scintillation detectors of X-rays, as well as other ionizing radiation. Tungsten ditelluride (WTe2) is used in the conversion of thermal energy into electrical energy. Even argon arc welding uses tungsten as an electrode.
Tungsten compounds are especially widely used. Composite materials and hard alloys based on tungsten carbide are needed for mechanical processing of both metals and non-metallic structures. This is especially necessary in mechanical engineering: milling, turning, chiselling, planing. It is now impossible to do without hard alloys when drilling wells and in the mining industry, and for this we need tungsten and molybdenum - production is mastering new technologies with their help.
Physical properties of Tungsten.
Tungsten crystallizes in a body-centered cubic lattice with a period a = 3.1647 Å; density 19.3 g/cm3, melting point 3410°C, boiling point 5900°C. Thermal conductivity (cal/cm·sec·°С) 0.31 (20°С); 0.26 (1300°C). Electrical resistivity (ohm cm 10-6) 5.5 (20°C); 90.4 (2700°C). Electron work function 7.21·10-19 J (4.55 eV), radiation energy power at high temperatures (W/cm2): 18.0 (1000°C); 64.0 (2200°C); 153.0 (2700°C); 255.0 (3030°C). The mechanical properties of Tungsten depend on previous processing. Tensile strength (kgf/mm2) for a sintered ingot 11, for pressure treated from 100 to 430; modulus of elasticity (kgf/mm1) 35000-38000 for wire and 39000-41000 for single-crystalline thread; Brinell hardness (kgf/mm2) for a sintered ingot 200-230, for a forged ingot 350-400 (1 kgf/mm2 = 10 MN/m2). At room temperature, tungsten is low-plastic.
Types of rare earth metal products
WS2 (tungsten sulfide) is a high-temperature lubricant that can withstand up to five hundred degrees Celsius. Where solid electrolyte is produced (high temperature fuel cells), tungsten trioxide is used. The textile and paint and varnish industries have significantly improved and complicated technologies using tungsten compounds as a catalyst and pigment in organic synthesis.
The industry produces a huge variety of products containing tungsten, molybdenum and other rare earth metals. The most common ones are electrodes, wire, tungsten powder, sheet and rod. Electrodes never melt and therefore can be used for welding high-alloy steels, non-ferrous metals and materials with different chemical compositions. No electrode will provide such high strength of the weld.
Mechanical properties of Molybdenum.
The mechanical properties of Molybdenum depend on the purity of the metal and its previous mechanical and thermal treatment. Thus, Brinell hardness is 1500-1600 Mn/m2, that is, 150-160 kgf/mm2 (for sintered rod), 2000-2300 Mn/m2 (for forged rod) and 1400-1850 Mn/m2 (for annealed wire); tensile strength for annealed wire is 800-1200 MN/m2. Elastic modulus Molybdenum 285-300 H/m2. Mo is more ductile than W. Recrystallizing annealing does not lead to brittleness of the metal.
Molybdenum
Molybdenum alloys and molybdenum itself are refractory materials. In its pure form, it is used in the form of wire or tape for heating devices - electric furnaces, even those operating in hydrogen at a temperature of 1600°C. Molybdenum tin and wire are needed in the electronics industry; they are also used in X-ray technology; various parts for X-ray tubes, electron tubes, and vacuum devices are made from molybdenum.
In addition, molybdenum, like tungsten, is widely used to improve steels. The molybdenum additive increases strength, hardenability, corrosion resistance, and toughness. Therefore, tungsten and molybdenum are used to create the most critical products and the most important parts. For hardness, stellites - chromium and cobalt - are introduced into such an alloy to fuse the edges of wear parts. Chromium, molybdenum, tungsten - such an alloy is almost impossible to erase. It is also given one of the first places among acid-resistant and heat-resistant alloys.
Chemical properties of Tungsten.
Under normal conditions, tungsten is chemically resistant. At 400-500°C, the compact metal noticeably oxidizes in air to WO3. Water vapor intensively oxidizes it above 600°C to WO3. Halogens, sulfur, carbon, silicon, boron interact with tungsten at high temperatures (fluorine with powdered tungsten - at room temperature). Tungsten does not react with hydrogen up to the melting point; with nitrogen above 1500°C forms nitride. Under normal conditions, Tungsten is resistant to hydrochloric, sulfuric, nitric and hydrofluoric acids, as well as aqua regia; at 100°C weakly interacts with them; quickly dissolves in a mixture of hydrofluoric and nitric acids. In alkali solutions when heated, tungsten dissolves slightly, and in molten alkalis with access of air or in the presence of oxidizing agents it dissolves quickly; in this case tungstates are formed. In compounds, tungsten exhibits a valence of 2 to 6; compounds with higher valence are the most stable. Tungsten forms four oxides: the highest - WO3 (tungsten anhydride), the lowest - WO2 and two intermediate ones W10O29 and W4O11. Tungsten anhydride is a lemon-yellow crystalline powder that dissolves in alkali solutions to form tungstates. When it is reduced with hydrogen, lower oxides and tungsten are successively formed. Tungstic anhydride corresponds to tungstic acid H2WO4 - a yellow powder, practically insoluble in water and acids. When it interacts with solutions of alkalis and ammonia, solutions of tungstates are formed. At 188°C, H2WO4 splits off water to form WO3. With chlorine, tungsten forms a number of chlorides and oxychlorides. The most important of them: WCl6 (mp 275°C, bp 348°C) and WO2Cl2 (mp 266°C, sublimates above 300°C), are obtained by the action of chlorine on tungsten anhydride in the presence of coal. With sulfur, tungsten forms two sulfides WS2 and WS3. Tungsten carbides WC (melt 2900°C) and W2C (melt 2750°C) are hard, refractory compounds; are obtained by the interaction of Tungsten with carbon at 1000-1500°C.
Space
An alloy of tungsten and molybdenum as part of the skin of the head part of any rocket and aircraft. In terms of strength, tungsten is in first place, molybdenum is in second place. However, specific strength at temperatures of about one and a half thousand degrees Celsius puts alloys with molybdenum in first place. If the temperatures are even higher, then tungsten and tantalum are invincible. Molybdenum is used to make the honeycomb panels of all flying spacecraft, the shells of capsules and rockets that return to Earth, heat exchangers, heat shields, covering the edges of wings, and stabilizers.
Where working conditions are difficult, rare earth metals help. From such a material one can expect high resistance to oxidation and gas erosion, high strength and the ability to withstand impact. Many parts of turbojet and rocket engines, tail skirts, turbine blades, injector flaps, control surfaces, rocket engine nozzles and so on - molybdenum copes with all these difficult jobs.
Physical properties of Molybdenum.
Molybdenum crystallizes in a cubic body-centered lattice with a period a = 3.14 Å. Atomic radius 1.4A, ionic radii Mo4+ 0.68A, Mo6+ 0.62A. Density 10.2 g/cm3 (20 °C); melting point 2620 °C; boiling point is about 4800 °C. Specific heat capacity at 20-100°C is 0.272 kJ/(kg K), that is, 0.065 cal/(g deg). Thermal conductivity at 20°C is 146.65 W/(m K), that is, 0.35 cal/(cm sec deg). Thermal coefficient of linear expansion (5.8-6.2)·10-6 at 25-700 °C. The electrical resistivity is 5.2·10-8 ohm·m, that is, 5.2·10-6 ohm·cm; electron work function 4.37 eV. Molybdenum is paramagnetic; atomic magnetic susceptibility -90·10-6 (20 °C).
On the ground
Promising materials for equipment that operate in phosphoric, sulfuric and hydrochloric acids are made from molybdenum and its alloys. It is stable even in molten glass, and therefore the glass industry widely uses molybdenum as melting electrodes.
Its alloys are used to make cores and molds for high-pressure casting of copper, zinc and aluminum alloys. Steels are processed with molybdenum under pressure - dies, dies, mandrels of piercing mills. Molybdenum also significantly improves the steel itself.
Obtaining Tungsten.
The raw materials for the production of Tungsten are wolframite and scheelite concentrates (50-60% WO3). Ferrotungsten (an alloy of iron with 65-80% tungsten), used in steel production, is directly smelted from the concentrates; To obtain tungsten, its alloys and compounds, tungsten anhydride is isolated from the concentrate. Several methods for producing WO3 are used in industry. Scheelite concentrates are decomposed in autoclaves with a soda solution at 180-200°C (a technical solution of sodium tungstate is obtained) or hydrochloric acid (a technical tungstic acid is obtained): 1. CaWO4 solid + Na2CO3 l = Na2WO4 l + CaCO3 solid 2. CaWO4 solid + 2HCl = H2WO4 solids + CaCl2 solution. Wolframite concentrates are decomposed either by sintering with soda at 800-900°C, followed by leaching of Na2WO4 with water, or by heating with sodium hydroxide solution. When decomposed by alkaline agents (soda or sodium hydroxide), a Na2WO4 solution contaminated with impurities is formed. After their separation, H2WO4 is separated from the solution. To obtain coarser, easily filtered and washed sediments, CaWO4 is first precipitated from a solution of Na2WO4, which is then decomposed with hydrochloric acid.) Dried H2WO4 contains 0.2 - 0.3% impurities. By calcining H2WO4 at 700-800°C, WO3 is obtained, and from it hard alloys are obtained. To produce metal Tungsten, H2WO4 is additionally purified using the ammonia method - dissolution in ammonia and crystallization of ammonium paratungstate 5(NH4)2O 12WO3 nH2O. Calcination of this salt produces pure WO3. Tungsten powder is obtained by reducing WO3 with hydrogen (and in the production of hard alloys, also with carbon) in tubular electric furnaces at 700-850°C. Compact metal is produced from powder using the metal-ceramic method, that is, pressing in steel molds under a pressure of 3000-5000 kgf/cm2 and heat treatment of pressed blanks - bars. The last stage of heat treatment—heating to approximately 3000°C—is carried out in special apparatus by directly passing an electric current through a rod in a hydrogen atmosphere. The result is Tungsten, which can be easily processed by pressure (forging, drawing, rolling, etc.) when heated. Tungsten single crystals are obtained from the rods using crucibleless electron beam zone melting.
